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after volatilization at 5 and 27 C for 30 and 7 days, respectively. The lighter
components disappeared from the sand and the sandy loam, and the residual
kerosene composition is similar regardless of temperature. The relative concen-
tration of the light fractions (C
10
-C
12
) diminished with time in all cases, while the
concentrations of the heavy fractions (C
14
and C
15
) increased. Galin et al. (
1990
)
determined the relationship between viscosity of the liquid and percent of kerosene
remaining in coarse, medium, and fine sand following volatilization. The highest
increase in viscosity occurs for coarse sand with an initial kerosene concentration
equivalent to the residual retention capacity, while the lowest increase occurs in
fine sand. Viscosity increases with the time of volatilization, in the order coar-
se [ medium [ fine sand, for each initial concentration.
These findings deal mainly with the volatilization of various components of
petroleum products that may be subject to nonideal equilibrium partitioning and
limitations of diffusion. An example of typical volatilization behavior of binary
NAPL mixtures entrapped at residual saturation, within partially saturated porous
media, is given by Abriola et al. (
2004
). They examine volatilization of binary
mixtures of styrene and toluene, and styrene and PCE, from columns packed
uniformly with aquifer grains of various mineral composition. The average sand
porosity, residual water, and residual NAPL saturation in sand columns were
0.382, 0.146, and 0.111, respectively. Various gas flow rates were used in a steady
state as well as in transient volatilization experiments, and component concen-
trations and mass transfer coefficients were determined. Abriola et al. (
2004
)
concluded that, under steady state conditions, NAPL saturation and composition
do not change appreciably. Under a low pore gas velocity, as in the transient case,
the initial component concentrations are proportional to their mole fractions. When
the more volatile components deplete, a sharp rise in the concentration and mole
fraction of the remaining component is observed, which may reach a near-satu-
ration level.
16.2 Biomediated Transformation of Contaminants
We examine two groups of contaminants that exhibit biologically mediated
transformation in the subsurface: petroleum hydrocarbons and pesticides. Both
aerobic and anaerobic pathways for their biodegradation are considered.
16.2.1 Petroleum Products
A hydrocarbon mixture may be biodegraded in both aerobic and anaerobic envi-
ronments. The literature on this subject is vast and deals with many specific
aspects; here, biologically mediated transformation of petroleum hydrocarbons is
discussed on the basis of the review by Cozzarelli and Baher (
2003
). Hydrocarbons
